A Gaussian surface is a hypothetical imaginary surface which can be used, along with Gauss’ Law, to calculate the electric field of a Continuous Charge Distribution.

Definition

A gaussian surface must be a closed, 3-dimensional surface. While it can be theoretically any such shape, a Gaussian surface is most useful if it matches the field symmetry. Why?

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The electric flux on on complex surfaces can be simplified if have the surface either being perpendicular or parallel to the electric field, completely. That is precisely what a gaussian surface intends to do.

Most commonly, cylindrical and spherical gaussian surfaces are used.